This invention concerns a temperature measuring apparatus and method. More particularly, it relates to an apparatus for measuring temperature in an electromechanical machine by a temperature detection probe using fiber optics.
The modern reality faced by the power generation industry is that equipment more and more is operated near its maximum current and voltage ratings. Along with these operating conditions comes an increase in unwanted heating of components. Reliability, maintenance and operation life of electrical equipment are directly affected by its operating temperature. When this temperature exceeds a certain value for any appreciable period of time, the life of the apparatus rapidly decreases.
Information as to "hot spots" and "over-temperature" conditions existing in the apparatus may indicate improper operation, defective parts, degradation of insulation, or even possible failure. Gathering that information, however, may prove difficult. Frequently conductors and equipment are at a high potential relative to ground. This high voltage, or its associated electromagnetic interference, hampers measurement of temperature directly on the conductors and makes use of metallic probes ill advised. For one reason, connections involving metallic conductors are susceptible to dangerous flashovers. Also, any currents induced in a metallic temperature sensor by the high potential could interfere with accurate temperature measurement.
It is desirable, therefore, to provide a device which incorporates a dielectric probe insertable in the electromechanical machine. It should be capable of continually measuring temperature during the operation of the machine and of effectively detecting places of excessive temperature within the machine, known as "hot spots". Such a dielectric probe should be immune to the effects of vibration so as to find its widest possible utility. In addition, it should be reusable without repair, in contradistinction to fuses, to minimize costs and service requirement.
Measuring the temperature is only the beginning of the problem. That information must then be communicated to the operator or control center for the particular machine. All too frequently the target area, whose temperatures are of interest, is buried deep within a labyrinth of electrical and mechanical components. It may be located in a most inaccessible location deep within the machine.
Previously many techniques have been used to measure temperature. The high potentials, or the other harsh aspects of the industrial environment, have rendered these prior attempts less than adequate.
One previous thermometer employed a common thermocouple to measure temperature changes within a power generator. This technique required extending the thermocouple's leads through the generator's housing. Thus a high potential conductor, with its associated bushings and insulation, was required just to monitor temperature. One approach to resolving this problem was the use of a radio transmitter. Connecting radio transmitters to the temperature sensing device, however, requires dealing with the presence of the electromagnetic field. The electromagnetic field interferes with signal transmission.
Non-metallic, non-magnetic temperature sensors have been previously sought. An avenue of solution explored has involved the use of fiber optics. For example, U.S. Pat. No. 3,960,017 uses fiber optics to optically read a thermometer, and transmit the information out of a transformer's winding. The temperature sensing column extends or contracts linearly, relative to the temperature, and across the opposing aperture of juxtaposed light pipes. This partially blocks light transmission which thereby makes light intensity proportional to temperature.
Another solution involves optic fibers in conjunction with a liquid crystal. This combination has been proposed for temperature measurement in biological research of living tissue. The liquid crystals become opaque with temperature changes, partially blocking transmitted light. The opacity is first calibrated against a standard thermometer, and then in operation, the probe containing the liquid crystals is illuminated and read by separate fiber optic light guides.
It is the case that certain properties of the light guide itself are affected by changes in temperature. As shall be discussed hereinafter, one of the properties, the ratio of the refractive indices of the materials from which the light guides are constructed, is both important for transmission and dependent on temperature.